Porous nickel hydroxide-manganese dioxide-reduced graphene oxide ternary hybrid spheres as excellent supercapacitor electrode materials.

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Porous Nickel Hydroxide−Manganese Dioxide-Reduced Graphene Oxide Ternary Hybrid Spheres as Excellent Supercapacitor Electrode Materials Hao Chen,†,‡ Shuxue Zhou,† and Limin Wu*,† †

Department of Materials Science and Advanced Materials Laboratory, Fudan University, Shanghai 20043, PR China School of Engineering, and National Engineering and Technology Research Center of Wood-based Resources Comprehensive Utilization, Zhejiang Agriculture and Forestry Unversity, Hangzhou Lin’an 311300, PR China

‡

S Supporting Information *

ABSTRACT: This paper reports the first nickel hydroxide− manganese dioxide−reduced graphene oxide (Ni(OH)2MnO2-RGO) ternary hybrid sphere powders as supercapacitor electrode materials. Due to the abundant porous nanostructure, relatively high specific surface area, well-defined spherical morphology, and the synergetic effect of Ni(OH)2, MnO2, and RGO, the electrodes with the as-obtained Ni(OH)2-MnO2RGO ternary hybrid spheres as active materials exhibited significantly enhanced specific capacitance (1985 F·g−1) and energy density (54.0 Wh·kg−1), based on the total mass of active materials. In addition, the Ni(OH)2-MnO2-RGO hybrid spheres-based asymmetric supercapacitor also showed satisfying energy density and electrochemical cycling stability. KEYWORDS: graphene, manganese dioxide, nickel hydroxide, ternary hybrid spheres, supercapacitor

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INTRODUCTION Supercapacitors, also known as electrochemical capacitors, have been widely applied in the fields of vehicles (especially electric vehicles, hybrid vehicles, and special heavy-duty vehicles), electric power, railways, telecommunications, and consumer electronics products.1−3 As the main contributors of electrical energy storage of supercapacitors, electrode materials are considered to be the current research focus in the energy storage field. In particular, the pseudocapacitive electrode materials based on transitional metal oxides and hydroxides have attracted increasing interest in recent years,4−10 because they have larger energy densities and specific capacitances with fast, reversible faradaic reactions, by at least 1 order of magnitude higher than those of carbon-based electrochemical double layer capacitors.1,11 However, due to their low specific surface area, or inappropriate (too high or too low) crystallinity, or poor electrolyte wettability, or weak electron transportation ability, lots of transitional metal oxides and hydroxides do not give full play to their capacitance performance.12 Therefore, how to improve the electrolyte access and electron transportation by reasonably designing structure and composition becomes the key strategy to develop high performance metal oxide/hydroxide pseudocapacitive materials. It is well-known that nickel- and manganese-based oxides/ hydroxides possess high theoretic specific capacitance, low cost, abundant sources, and environmentally friendly nature.3,13−16 For example, Ozkan et al. fabricated a MnO2 nanowire based © XXXX American Chemical Society

hybrid nanostructured foam, which exhibited excellent specific capacitance (1109 F g−1).17 Dai et al. reported a Ni(OH)2 based hybrid material as the electrode with very high specific capacitance of 1335 F g−1.18 Very recently, we have reported a novel one-step method for synthesizing Ni(OH)2-MnO2 hybrid nanosheets.19 The as-prepared hybrid nanosheets in situsupported on nickel foam as a supercapacitor electrode exhibited ultrahigh specific capacitance (Cs, 2628 F·g−1 at 3 A·g−1). However, most of this related research focuses on the nickel- and manganese-based oxides/hydroxides binary composites as electrode materials to date; little involves nickel− manganese−graphene based ternary hybrid structures for supercapacitors. In this study, we successfully synthesize the first nickel hydroxide−manganese dioxide−reduced graphene oxide (Ni(OH)2-MnO2-RGO) ternary hybrid sphere powders, through a one-step hydrothermal codeposition of graphene oxide (GO) and the precursors of nickel hydroxide and manganese dioxide. Benefitting from the abundantly porous nanostructure, relatively high specific surface area, well-defined spherical morphology, and the synergetic effect of Ni(OH)2, MnO2, and RGO, these novel Ni(OH)2-MnO2-RGO ternary hybrid spheres as the supercapacitor electrode material exhibit significantly enhanced specific capacitance (1985 F·g−1) and Received: March 11, 2014 Accepted: May 5, 2014

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dx.doi.org/10.1021/am5014375 | ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

ACS Applied Materials & Interfaces

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Figure 1. Typical SEM images of (a) the Ni(OH)2-MnO2-RGO hybrid spheres and (b) the hybrid spheres at higher magnification, in which the inset is the image of hybrid nanosheets. (c) SEM elemental mappings of the hybrid sphere. Typical TEM images of (d) the hybrid sphere (the inset is the SAED pattern of the whole hybrid sphere) and (e) a typical sphere edge at higher magnification (GO suspension: 0.20 g).

energy density (54.0 Wh·kg−1), based on the total mass of active materials. These values are obviously higher than those of most previously reported nickel−manganese oxide/hydroxide, manganese dioxide−graphene, nickel oxide/hydroxide−graphene, and typical nickel−cobalt binary oxide/hydroxide− graphene composites. In addition, the Ni(OH)2-MnO2-RGO ternary hybrid sphere-based asymmetric supercapacitor with porous freeze-dried RGO as the negative electrode material also exhibits satisfying energy density and electrochemical cycling stability.

from Tianjin Ebory Chemical Co., Ltd. (China). All reagents were used as received. Graphene oxide (GO) was prepared by our previously reported method.20 Cationic gemini surfactant G12 (C12H25N(CH3)2(CH2)6(CH3)2NC12H25Br2) was synthesized according to our previous report.19 Synthesis of Hybrid Materials. The Ni(OH)2-MnO2RGO hybrid material was prepared by a simple one-step process as follows: Typically, a solution of Ni(NO3)2·6H2O (0.34 mmol), MnCl2·4H2O (0.51 mmol), and methanol (4.0 g) was slowly added into the magnetically stirred homogeneous suspension containing GO aqueous suspension (0.20 g, 1.44 wt %), H2O (3.0 g), and methanol (8.0 g). The as-obtained suspension was transferred to a 100 mL Teflon autoclave. The autoclave was then sealed and kept at 180 °C for 24 h to allow the growth of Ni(OH)2-MnO2-RGO hybrid material, through the following reactions: (i) methanol and NO3− undergo a redox reaction releasing hydroxyl ions under 180 °C: 4CH3OH + NO3− → 4HCHO + NH3 + OH− + 2H2O;21,22 (ii) the Ni2+

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EXPERIMENTAL SECTION Materials. Hydrochloric acid (37%, HCl) was purchased from Shanghai Chemical Reagent Co., Ltd. (China). Potassium hydroxide (KOH), nickel nitrate (Ni(NO3)2·6H2O), manganese chloride (MnCl2·4H2O), acetone, anhydrous ethanol, and methanol were purchased from Sinopharm Chemical Reagent Corp (China). Acetylene black (F-900, AB) was purchased B



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Figure 2. (a) EDX spectrum of hybrid spheres. (b) Ni 2p and (c) Mn 2p XPS scans of the hybrid spheres. (d) Raman spectra and (e) XRD patterns of RGO, Ni(OH)2-MnO2, and Ni(OH)2-MnO2-RGO powders. (f) Nitrogen (77 K) adsorption/desorption isotherms and BJH pore size distribution of as-synthesized Ni(OH)2-MnO2 and Ni(OH)2-MnO2-RGO powders.

ions from the Ni(NO3)2 solution and Mn2+ ions from the MnCl2 solution react with OH− ions to form Ni(OH)2 and Mn(OH)2; and (iii) the Mn(OH)2 converts to stable MnO2 upon reaction with oxygen. GO is reduced to RGO by reacting with methanol at high temperatures.23 After being cooled to room temperature, the resultant product was centrifuged and washed with H2O and ethanol several times to remove surface ions and molecules, followed by being dried at 80 °C for 12 h to remove the adsorbed solvents. For the sake of comparison, the hybrid materials with surfactant G12 to replace RGO, or without using Mn2+, or different GO feeding amounts (the total water amount was kept at 3.20 g) were also fabricated using the same procedure. Fabrication of Electrodes. Nickel foam was first pretreated successively with acetone, 2 M HCl solution, absolute ethanol, and deionized water, each for 15 min, to ensure a clean surface. The freeze-dried reduced graphene oxide (FRGO) was produced according to our previous report.19 Hybrid- and FRGO-electrodes were fabricated as follows: a mixture of hybrid (or FRGO) powders, 23 wt % of acetylene black (as the electrical conductor), 5 wt % of polytetrafluorene−ethylene (as the binder), and a small amount of water was prepared by milling to produce a homogeneous paste. This paste was then pressed onto nickel foam current-collectors to produce hybrid and FRGO electrodes. The loading densities of active materials were about 6 mg·cm−2 for all electrodes.

Characterization. The morphologies were observed by scanning electron microscopy (SEM, S-4800, Hitachi), and the TEM images were obtained on a Philips CM200FEG field emission microscope. The crystalline structure was characterized by XRD patterns recorded in a Rigaku D/max-kA diffractometer with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS, PHI 5000C ESCA System), energydispersive X-ray spectroscopy (EDX, TSL, AMETEK), and Raman spectroscopy (Jobin Yvon LabRam-1B) measurements were employed to investigate the compositions of the samples. An automated adsorption apparatus (Micromeritics, ASAP 2010) was used to analyze the surface characteristics of the samples using gas physisorption at 77 K. Electrochemical Measurement. The electrochemical properties of the as-obtained hybrid powder- and the FRGObased electrodes were investigated under a three-electrode cell configuration at 25 °C in 1 M KOH. The electrodes were soaked in a 1 M KOH solution and degassed in a vacuum for 5 h before the electrochemical test. Platinum foil and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. The electrochemical properties of the asymmetric supercapacitor were investigated under a two-electrode cell configuration with Ni(OH)2-MnO2-RGO hybrid powder as the positive electrode material and FRGO as the negative electrode material in 1 M KOH electrolyte solution. The cyclic voltammetry (CV), galvanostatic charge− C



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Scheme 1. Possible Formation Mechanism of Ni(OH)2-MnO2-RGO Hybrid Sphere and the Structural Schematic Diagram of a Single Nanosheet in a Hybrid Sphere

spectrum displays two characteristic peaks at 642.3 and 653.9 eV, corresponding to the Mn 2p3/2 and Mn 2p1/2 spin−orbit peaks of MnO2 (Figure 2c), confirming the existence of Ni(OH)2 and MnO2 in the hybrid spheres.26,27 The XPS scan of pure RGO prepared by a similar hydrothermal process showed a decreasing O/C ratio from 0.38 to 0.23 (Figure S1e), and its Raman spectrum demonstrated an increasing D/G band intensity ratio from 1.07 to 1.15 (Figure S1f) after the hydrothermal reaction. These results indicate that GO has been reduced to RGO by this hydrothermal treatment.28,29 Moreover, the comparison of C 1s XPS spectra of GO and RGO further revealed that the decrease of the O/C ratio was mainly caused by the proportional reduction of ester and carbonyl groups (Figure S1g). The Raman spectrum of hybrid sphere powders displays a D band at 1335 cm−1, a G band at 1585 cm−1 for RGO, and another band from 540 to 670 cm−1 for the Ni−O and Mn−O stretching vibrations of Ni(OH)2 and MnO2 (Figure 2d).30,31 The XRD patterns show that the well-defined diffraction peaks of Ni(OH)2-MnO2-RGO can be successfully indexed to the αNi(OH)2 phase (JCPDS 38-0715), γ-MnO2 phase (JCPDS 140644), and the AB stacking order of graphene sheets (10.4°, corresponding to interlayer spacing of 0.85 nm, Figure 2e),29 which is similar to that of our previously reported Ni(OH)2MnO2 hybrid nanosheets.19 The as-obtained hybrid sphere powder appeared black, different from the brown color of Ni(OH)2-MnO2 binary powder (Figure S1h). These results indicate that the as-obtained hybrid spheres are composed of αNi(OH)2, γ-MnO2, and RGO, and the Cl− ions may act as the intercalated counteranions of layered α-Ni(OH)2 based on the above EDX result. In addition, the Ni(OH)2-MnO2-RGO ternary hybrid sphere has the BET and Langmuir surface areas of 147 and 185 m2 g−1, respectively, which are almost double those of the Ni(OH)2-MnO2 (76 and 96 m2 g−1, respectively), as indicated by Figure 2f. Possible Formation Mechanism of Hybrid Spheres. The similar diffraction peaks between Ni(OH)2-MnO2-RGO and Ni(OH)2-MnO2, as shown in the XRD pattern (Figure 2e), suggest that the nanosheets in hybrid spheres form through a mechanism similar to the preferentially oriented growth of Ni(OH)2 nanosheets described in previous reports.19,32,33 As shown in Scheme 1, due to the electrostatic interaction, most Ni2+ and Mn2+ ions are located on the surfaces of GO sheets

discharge, and electrochemical impedance spectroscopy (EIS) measurements were conducted on a CHI 660E electrochemical workstation (Shanghai CH Instrument Company, China).

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RESULTS AND DISCUSSION Morphology and Composition of Hybrid Spheres. The typical SEM image of the as-obtained hybrid powders, as shown in Figure 1a, presents a spherical feature with diameters of 1−5 μm, and these hybrid spheres possess a porous flowerlike hierarchical structure composed of nanosheets with dozens of nanometers thick (Figure 1b). This hierarchical structure facilitates the charge transport and ion diffusion. SEM elemental mappings (Figure 1c) reveal a uniform distribution of Ni, Mn, O, and Cl elements in the as-obtained sphere (the existence of C element cannot be determined due to the effect of carbon conductive adhesive). This indicates a relatively uniform chemical composition of the as-obtained hybrid sphere. The TEM images of the as-obtained hybrid sphere further display a rough and transparent edge (Figure 1d), indicating an ultrathin nature. The selected-area electron diffraction (SAED) pattern of the hybrid sphere reveals two well-defined diffraction rings (inset of Figure 1d), suggesting its polycrystalline nature. The energy-dispersive X-ray (EDX) spectra of the entire area and center area of the hybrid sphere (Figure S1a and b in the Supporting Information) show the similar element composition of Ni, Mn, O, Cl, and C elements, except for the Cu signal from the Cu grid. A highermagnification TEM image of a typical sphere edge clearly shows that this hybrid sphere consists of ultrathin nanosheets, which are composed of several thinner layers (Figure 1e). The EDX spectrum of the edge region of the hybrid sphere (Figure S1c) suggests that these thinner layers may be the stacked structure of Ni(OH)2, MnO2, and graphene layers (the absence of Cl element may be attributed to the ion migration of Cl−). The EDX spectrum (Figure 2a) and the full XPS spectrum (Figure S 1d) of the as-prepared hybrid powder sample further prove the basic element composition of as-obtained spheres, Ni, Mn, O, C, and Cl elements, which should be contributed by the composition of Ni(OH)2, MnO2, and RGO. The Ni 2p XPS spectrum is found to have two shakeup satellites (indicated as “Sat”) close to two spin−orbit doublets at 873.2 and 855.6 eV (Figure 2b). These are here given as Ni 2p1/2 and Ni 2p3/2 signals of Ni(OH)2, respectively.24,25 The Mn 2p XPS D



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Figure 3. SEM images of the (a) Ni(OH)2-MnO2-G12 and (b) nickel hydroxide−RGO powders.

Figure 4. SEM images of the Ni(OH)2-MnO2-RGO powders with different GO suspension amounts: (a) 0.00, (b) 0.05, (c) 0.10, (d) 0.40, (e) 0.60, (f) 0.80, and (g) 1.60 g. (h) SEM image of the pure RGO.

and first react with OH− ions to produce primary particles. These primary particles aggregate into chains which partly deposit on the surfaces of GO to become the cores of more

amorphous primary particles aggregating. As the primary particles continue to aggregate, they begin to crystallize and gradually form primary nanosheets. During this process, E



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Figure 5. (a) Comparison of CV curves of Ni(OH)2-MnO2-RGO hybrid sphere-electrode at different scan rates. (b) Galvanostatic discharge curves of Ni(OH)2-MnO2-RGO hybrid sphere-electrode at different current densities. (c) Comparison of specific capacitances of the hybrid electrodes prepared with different GO amounts. (d) Comparison of Nyquist plots of Ni(OH)2-MnO2-RGO, Ni(OH)2-MnO2, and RGO electrodes.

may be a result of excessive graphene wrapping the forming hybrid nanosheets and destroying their self-assembly ability. For pure RGO, due to the attractive forces between layers during the drying process,39,40 only high aggregates were observed (Figure 4h). Electrochemical Properties of Hybrid Sphere Electrodes. Figure 5a demonstrates the typical cyclic voltammetry (CV) curve of the as-obtained hybrid spheres, which was fabricated at a 0.20 g GO suspension feeding amount, as an electrode for a supercapacitor at different scan rates. A pair of well-defined redox peaks within 0−0.5 V can be ascribed to a faradaic redox reaction related to M−O/M−O−OC (M represents Ni or Mn, C represents H or K).41,42 Figure 5b shows the galvanostatic charge−discharge curves of the electrode based on the as-obtained hybrid spheres at different current densities. Both indicate the strong pseudocapacitive nature of the as-obtained electrodes. The specific capacitances of the Ni(OH)2-MnO2, pure RGO, and the ternary hybrid spheres prepared at different GO feeding amounts are shown in Figure 5c. Most hybrid sphere-electrodes possess significantly higher specific capacitances than the Ni(OH)2-MnO2 or RGO electrodes. This can be attributed to the synergistic effects of the Ni(OH)2-MnO2-RGO hybrid sphere as follows: (i) The asobtained hybrid spheres are composed of Ni(OH)2-MnO2 and RGO. Both of them can contribute their capacitance. (ii) The intercalation of graphene sheets can improve the electron transportation from hybrid sphere structures to the current collector, which can be confirmed by the decrease in equivalent series resistance (Rs, the real axis intercept), as shown in Figure 5d. (iii) The intercalation of graphene sheets can suppress the excessive self-aggregation of hybrid nanosheets and lead to visible increasing of specific surface areas as indicated by Figure 2f. This promotes better electrolyte access within the electrode materials and more efficient exposure of active sites to the electrolyte. As the GO content increases, as shown in Figure 5c, the Cs increases. However, excessive GO, e.g., 0.40 g, decreases the Cs

divalent manganese is converted to tetravalent manganese by reacting with dissolved oxygen,34,35 and GO is reduced to RGO by reacting with methanol at high temperatures.23 Because of the hydrogen-bonding interactions between MnO2 and Ni(OH)2 primary nanosheets, MnO2 primary nanosheets may disorderly stack with Ni(OH)2 primary nanosheets and grow along with RGO to form a hybrid nanosheet (Scheme 1). These nanosheets tend to aggregate perpendicular to the surface planes to decrease the surface energy by reducing exposed areas. Therefore, as the reaction proceeds, a spontaneous energy-minimizing self-assembly of the resulting hybrid nanosheets would occur, producing spherical assembled structures.36,37 And because the intercalation of graphene sheets can weaken the excessive oriented growth and the aggregation of hybrid nanosheets, the Ni(OH)2-MnO2-RGO nanosheets finally self-assemble into a spherical shape (Figure 1a and b), rather than some irregular structures like the Ni(OH)2-MnO2 binary hybrid materials with surfactant G12 as the structure growth assisting agent (called Ni(OH)2-MnO2G12 here, as shown in Figure 3a). And due to the too high oriented growth of pure nickel hydroxide, the nickel hydroxideRGO without MnO2 does not self-assemble into a spherical structure (Figure 3b). Thus, the as-obtained Ni(OH)2-MnO2RGO ternary hybrid hierarchical structure has high surface area, a basal plane, and edge surface and may possess enhanced electrochemical properties.38 The Effects of GO Amounts. The GO amount also plays an important role in the morphological control of the hybrid spheres. Without GO, the as-obtained hybrid spheres consisted of relatively larger nanosheets and worse spherical structure (Figure 4a). This should be a result of the relatively high oriented growth and aggregation ability of pure Ni(OH)2MnO2. As the GO amount increased, the assembled structures increased in size and then began to become round (Figures 4b−d and 1a,b). However, further increasing the GO amount led to the disintegration of spherical structure, and even the self-aggregation of graphene nanosheets (Figure 4e−g). This F



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Figure 6. (a) Comparison of specific capacitances of Ni(OH)2-MnO2-RGO, Ni(OH)2-MnO2-G12, and nickel hydroxide-RGO electrodes. (b) Comparison of Nyquist plots of Ni(OH)2-MnO2-RGO and Ni(OH)2-MnO2-G12 electrodes. (c) Nitrogen (77 K) adsorption/desorption isotherms and BJH pore size distribution of as-synthesized Ni(OH)2-MnO2-G12 and Ni(OH)2-MnO2-RGO powders. (d) XRD patterns of Ni(OH)2-MnO2RGO, Ni(OH)2-MnO2-G12, and nickel hydroxide-RGO powders.

Figure 7. (a) Comparison of energy density vs power density curves of Ni(OH)2-MnO2-RGO hybrid electrode and Ni(OH)2-MnO2-RGO//FRGO asymmetric supercapacitors. (b) CV curves of the asymmetric supercapacitor at different scan rates. (c) Galvanostatic charge−discharge curves of the asymmetric supercapacitor at different current densities. (d) Cycling performance of the asymmetric supercapacitor a current density of 8 A g−1.

the rate capability is not impressive, the obtainable highest Cs (1985 F·g−1 at 2 A·g−1) is obviously higher than the specific capacitances of most previously reported nickel−manganese oxide/hydroxide, manganese dioxide-graphene, nickel oxide/ hydroxide-graphene binary, and nickel−cobalt oxide/hydroxide-graphene composites (Table S1). This high specific capacitance is believed to be caused by the unique nanostructure features of the present Ni(OH)2-MnO2-RGO hybrid spheres: (i) An abundant porous nanostructure, relatively high specific surface area, and well-defined spherical morphology allow easy electrolyte access and promote fast ion

due to the disintegration of spherical structure and the selfaggregation of graphene nanosheets as indicated by Figure 4. The excessive self-aggregation of graphene can produce higher Rs (Figure 5d), which inhibits the capacitance output. The hybrid spheres formed at 0.20 g of GO show the highest Cs values, being 1985, 1736, 1501, 1062, and 724 F·g−1 at current densities of 2, 3, 4, 6, and 8 A·g−1, respectively. The modest decrease in Cs at higher current densities may be due to the limited ion migration into the inside of active materials, as the current density increased. This can be further improved by decreasing the loading densities of active materials. Although G



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FRGO as electric double layer capacitive material. The obtainable highest energy density and corresponding average power density are comparable to those of some other asymmetric supercapacitors49−53 (Table S3). We also used galvanostatic charge−discharge measurement to evaluate the capacitance durability and Coulombic efficiency of the asfabricated asymmetric supercapacitor. As shown in Figure 7d, the as-obtained asymmetric supercapacitor can still retain about 75% of its original capacitance even after 2000 cycles, which is comparable to some nickel- and manganese-based asymmetric supercapacitors.27,54,55 The capacitance durability can be further improved when a capsulation cell is employed to prevent the active materials from loosening or flaking off. Furthermore, it is noted that the Coulombic efficiency can be maintained at about 98% except the previous several cycles. These results demonstrate that our asymmetric supercapacitor based on Ni(OH)2-MnO2-RGO//FRGO also possesses good electrochemical stability.

diffusion. (ii) The synergistic effect between Ni(OH)2 and MnO2 in hybrid spheres can improve the pseudocapacitance output of them, like our previously reported Ni(OH)2-MnO2 hybrid nanosheets.19 (iii) The intercalation of graphene sheets improves the electron transport of binary metal oxides/ hydroxides and thus enhances the synergistic effect of hybrid materials. Figure 6a displays that the Ni(OH)2-MnO2 binary hybrid nanosheet powder (Ni(OH)2-MnO2-G12), prepared with surfactant G12 as the structure growth assisting agent, exhibits higher Cs (1402 F·g−1 at 3 A·g−1) than that of pure Ni(OH)2MnO2 powder (1121 F·g−1 at 3 A·g−1), indicating that G12 can also improve the Cs of hybrid nanostructure powders. However, this increase is inferior to that endowed by RGO. This is because the intercalation of RGO can decrease the electric resistance and increase the specific surface area of active materials compared to G12, as shown in Figures 2f, 5d, and 6b,c. Figure 6d demonstrates that the nickel hydroxide-RGO binary hybrid synthesized without using Mn2+ possesses a Ni3(NO3)2(OH)4 phase (JCPDS 22-0752), suggesting that the addition of GO has no influence on the crystal transition of nickel hydroxide, unlike the addition of Mn2+ (Figure 2e). And the nickel hydroxide-RGO has obviously lower Cs than the Ni(OH)2-MnO2-RGO ternary hybrid. These results indicate that the present ternary hybrids have better capacitance performances than binary counterparts. On the basis of the obtainable specific capacitance values and the reported method of calculation,43,44 the energy densities of the as-obtained Ni(OH)2-MnO2-RGO ternary hybrid sphereelectrode at 0.20 g of GO were further calculated to be 54.0, 39.9, 29.2, and 16.0 Wh·kg−1 at average power densities of 392, 497, 561, and 653 W·kg−1, respectively (Figure 7a). Although the average power densities were not impressive due to a relatively weak electric double layer capacitive nature of Ni(OH)2-MnO2-RGO, the maximum energy density (54.0 Wh·kg−1 at 392 W·kg−1) was found to exceed those of some recently reported nickel−manganese oxide/hydroxide-,45,46 manganese dioxide-graphene-,44,47 and nickel oxide/hydroxide-graphene-powder-based composite supercapacitor electrodes,18,48 as shown in Table S2. Electrochemical Properties of Ni(OH)2-MnO2-RGO// FRGO Asymmetric Supercapacitors. In order to evaluate the possibility of using the as-obtained Ni(OH)2-MnO2-RGO hybrid spheres to construct a practical supercapacitor device, we further fabricated an asymmetric supercapacitor with Ni(OH)2-MnO2-RGO hybrid spheres and porous freeze-dried reduced graphene oxide (FRGO) powders as the positive and negative electrodes materials, respectively. Figure 7b and c demonstrate that the as-fabricated Ni(OH)2-MnO2-RGO// FRGO asymmetric supercapacitor exhibits very good capacitive behavior at a potential window of 0−1.6 V. The highest energy density of the asymmetric supercapacitor can be found to be 32.6 Wh·kg−1 at an average power density of 305 W·kg−1, derived from its discharge curves. At a high power density of 1016 W· kg−1, the energy density can still remain at 11.5 Wh· kg−1 (Figure 7a). Compared with Ni(OH)2-MnO2-RGO electrode, this asymmetric supercapacitor displays a much milder attenuation in energy density due to the limitation of relatively low capacitance output of the FRGO negative electrode (281 F·g−1 at 0.5 A·g−1). However, the obtainable highest power density has a significant enhancement (Figure 7a), because of the relatively high power output nature of

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CONCLUSIONS In this work, highly porous ternary hybrid spheres composed of α-Ni(OH)2, γ-MnO2, and RGO were successfully fabricated using a facile one-step hydrothermal codeposition method. Due to the abundant porous nanostructure, relatively high specific surface area, well-defined spherical morphology, and the synergetic effects of Ni(OH)2, MnO2, and RGO, the electrodes with the as-obtained Ni(OH)2-MnO2-RGO hybrid sphere powders as active materials exhibited very high specific capacitance (1985 F·g−1) and energy density (54.0 Wh·kg−1). Both were significantly higher than those of most previously reported nickel−manganese oxide/hydroxide, manganese dioxide-graphene, nickel oxide/hydroxide-graphene binary, and typical nickel−cobalt oxide/hydroxide-graphene ternary composites. In addition, the Ni(OH)2-MnO2-RGO ternary hybrid sphere-based asymmetric supercapacitor exhibited comparable energy density and electrochemical cycling stability to some nickel- and manganese-based asymmetric supercapacitors. This suggestes that the as-prepared Ni(OH)2-MnO2-RGO ternary hybrid spheres are promising electrode materials to fabricate high performance supercapacitors by further optimized capsulation. And the present method offers a promising design and synthetic protocol of electrode materials for future supercapacitor applications.

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ASSOCIATED CONTENT

S Supporting Information *

EDX spectra of the entire area, center area, and edge region of hybrid sphere; full XPS and C 1s XPS spectra of GO and RGO; full XPS spectra of Ni(OH)2-MnO2-RGO; Raman spectra of GO and RGO; photographs of as-obtained Ni(OH)2-MnO2 and Ni(OH)2-MnO2-RGO hybrid spheres powders. Comparison of the maximum Cs, maximum energy densities, and corresponding average power densities of some reported nickel−manganese oxide/hydroxide-, manganese dioxide-, graphene-, nickel oxide/hydroxide-graphene-, and nickel− cobalt oxide/hydroxide-graphene-based composite pseudocapacitive materials and the present work; comparison of the maximum energy densities, corresponding average power densities, and voltage range of some reported nickel or manganese oxide/hydroxide based asymmetric supercapacitors and the present work. This material is available free of charge via the Internet at http://pubs.acs.org. H



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial supports were received from the National Natural Science Foundation of China (Grants 51133001 and 21374018), National “863” Foundation, Science and Technology Foundation of Ministry of Education of China (20110071130002), Science and Technology Foundation of Shanghai (12 nm0503600, 13JC1407800), Natural Science Foundation of Zhejiang Province of China (LQ14E030011), Zhejiang innovative group (2012R10023-16), BURC research foundation (2013SWZ01-2), and talents foundation (2011FR041, 2013FR066) from Zhejiang A&F University.

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